docs(expenses): define ledger maintenance rules

AGENTS.md

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+# Repository Maintenance Rules
+
+## Expense synchronization
+
+Whenever a document is added or changed and it confirms an actual booked or paid expense, update [`expenses.md`](expenses.md) in the same change.
+
+1. Add or update exactly one ledger row per real expense. Do not duplicate an expense merely because it appears in both a booking file, PDF, email, or itinerary.
+2. Do not add shortlist prices, rejected options, refundable deposits, estimated optional fees, or TBC amounts as expenses.
+3. Record the payer exactly. **If it is not clear who paid, ask the user before updating the ledger; do not infer the payer.**
+4. Preserve the source currency in the ledger. For shared-cost settlement, use the USD amount recorded in Tricount.
+5. The accounting couples are **KTVT** (KT + VT) and **LKMS** (LK + MS). Shared expenses are split 50/50 between them unless a source clearly identifies an expense as couple-only.
+6. **Couple-only expenses** stay in the ledger for completeness but do not affect the cross-couple settlement.
+7. Ignore discrepancies below US$1 when they plausibly result from currency conversion or rounding.
+8. After every ledger-row change, recompute the net balance and update the single settlement sentence in `expenses.md`. Show only the final result in the form: **“KTVT owes LKMS US$X.XX.”** (or the reverse).